1. Field of the Invention
The present invention relates to arrays of conducting microelectrodes and particularly to arrays that have metallic, high-aspect-ratio microelectrodes, high surface area, surfaces that can be ground and polished to nonplanar shapes, and compatibility with biological tissue.
2. Description of the Related Art
Microelectrode arrays are used to deliver or detect (stimulate or record) electrical signals at discrete, spatially resolved locations. Microelectrode arrays are desirable for use in a number of diverse applications, including, for example, stimulation and recording of neural signals in a neural prosthesis, stimulation of retinal signals in a retinal prosthesis and detection of chemical potentials in an electrochemical sensor.
Low-aspect-ratio microelectrode arrays are fabricated using conventional silicon-based microfabrication techniques. These techniques utilize standard silicon processing methods such as photolithography, to yield arrays of thin films of metallic or carbon electrodes on a silicon substrate. Thin film, microfabricated microelectrode arrays often have limited stability and useful lifetime as a result of defects present in the various layers of the array. These defects lead to poor resistance to corrosion and subsequent swelling and delamination of the layers. Microfabricated arrays typically are fragile and cannot be cleaned using conventional cleaning methods and materials (polishing, solvents, sonication), but must be cleaned using methods such as reactive ion etching. Arrays of high aspect ratio, conducting microelectrodes have been of interest in neurobiology. These microelectrode arrays are designed to penetrate brain tissue to permit highly localized electrical stimulation and/or recording of signals from neural tissue. Silicon micromachining, silicon microfabrication and techniques involving bundling of multiple solid wires have been used to fabricate arrays of electrodes that are capable of penetrating neural tissue. There are advantages and disadvantages associated with each approach. Micromachined electrodes are limited in number (˜10 to ˜100) and are coated with a layer of platinum at the tip. These platinum coatings can crack due to mechanical stress or corrosion. Cracks can lead to contamination problems, delamination and the appearance of non-ohmic interfaces causing degradation in the performance of the electrodes. Although solid wire electrode arrays do not delaminate and will not exhibit non-ohmic interfaces, they typically have only a handful of electrodes. High-aspect-ratio microelectrode arrays are also of interest for their use as an electrical interface in an intraocular retinal prosthesis (IRP). An IRP is a device that is attached directly to the retina and that is intended to electrically stimulate the retina in an effort to restore vision to patients with impaired vision. An array of high-aspect-ratio microelectrodes is necessary to conduct the electrical stimulation from a flat microelectronic circuit to the curved surface of the retina.
Arrays of magnetic nanowires have been grown in nanochannel glass substrates using electrodeposition. The diameter of the magnetizable nanoposts ranged from 10-1000 nm. The arrays were made by electrodeposition of magnetizable material from plating solutions into the channels of a nanochannel glass template. These are nanocomposite materials that feature large numbers of densely packed, high-aspect-ratio, magnetic nanowires (up to 1012/cm2 and claimed aspect ratios up to 10,000).
Nguyen and Tonucci in U.S. Pat. No. 6,185,961 taught a method for the manufacture of nanocomposite materials involving the electrodeposition of metal within the channels of nanochannel glass. The nanowire arrays taught by Nguyen are extremely small and are not well suited for electrode applications. This is due to the small size of the individual electrodes, the small overall size of the array and the limitation on the overall length of the nanowires. In addition, nanowire arrays can not be used as implantable electrodes because the wires are not long enough, nor are they strong enough to penetrate tissue. Further, the methods taught by Nguyen for the manufacture of nanowire arrays do not work for the deposition of wires having diameter greater than a micron. For example, the deposition art taught by Nguyen required occluding the ends of the nanochannels with a layer of sputtered metal. This approach cannot be used for microchannel samples because the channels are too big to occlude. The procedures taught in this disclosure for deposition in larger channels eliminate the need for occluding the channels.
Previous art for the electrodeposition of nanowire arrays (Nguyen) teaches deposition at constant voltage. Previous art (Nguyen) for the electrodeposition of nanowire arrays was limited to samples of extremely small surface area. This invention permits electrodeposition within microchannel glass templates without damage to the glass wafers. The methods taught in this disclosure also allow the deposition of metals with improved bio-compatibility and lower electrical impedance.